As is well known in ground reinforcement and in particular ground reinforcement in the mining industry, there are numerous apparatus and methods used in rock bolting for strata control. An internationally accepted method of strata control is resin-anchored rebar or bolts. Typically, the rebar's surface is patterned or deformed to form a better mechanical interlock or bond between the bar, resin and the rock mass. These reinforcing methods are intended to react to rock mass movement, develop a restraining force and transfer that force back to the rock mass.
Typically, resin anchored bolts require a borehole to be drilled in the ground reinforcement, such as for example a mine roof or wall. A resin cartridge is inserted into the borehole and then the bolt is inserted and rotated to rupture the resin cartridge.
Rebar typically is between four (4) and eight (8) in length to provide the necessary length to penetrate into the rock mass and can provide at least ten (10) inches for attaching fasteners etc. In order to insert the rebar into a bore hole, the working space typically must be higher than the length of the bolt. In some applications, the working space is less than six (6) feet. This is common in mining operations when the coal seam narrows. To accommodate for lower seam underground mining heights, where the required penetration depth is higher than the existing mine opening height; the bars have to be bent during or prior to the installation process. As is known to those of ordinary skill in the art, the rebar has to be bent manually by the roof bolt operator. To facilitate bending, the rebar is deformed to a narrow section close to the middle of the bar. The narrow section allows the rebar to be bent with less force than would be required if there were no narrowed section.
It is difficult, if not impossible, to bend rebar larger than no. 5 (⅝-inch diameter). Traditionally, to facilitate bending of the bar, the bar is mechanically modified by heating the bar in a separate process and stamping it with a high impact hydraulically operated ram. This process, called “Hot-Notching”, consists of a specially designed heating unit for gas or induction heating. The rebar is placed on a conveyor with the center portion of the bar being exposed to the heat source and the remaining end portions of the bar not being exposed to heat. The conveyor moves the bar through at a predetermined variable speed (controlled by sensors) so the bar is the correct temperature. The heated rebar is then hydraulically stamped to flatten the center section. This process doesn't remove any material from the roof bolt; it simply moves the material to a flattened position to facilitate bending.
The problem with this method is that it requires very costly machinery, is very labor intensive, is very costly, and the heating and deformation changes the steel property characteristics.
There is also a mechanical method for reducing the thickness of the bar diameter by removing a section of the steel material. This process is completed by feeding the bolts into a hydraulic machine that “grinds” the material into the predetermined form and at the location where the bends are required (every mine can have a different specification on the location of the bend(s) based on mining height and machine operator preference. The final web-thickness is critical for regulatory requirements for minimal bolt strength.
As with the previous method, this method also suffers from the problem that it requires costly machinery, is labor intensive, is very costly, and the deformation changes the steel property characteristics.
As described, the hot-notch or mill-notch processes requires considerable labor, expensive equipment and costly heat energy. Additionally, the Regulatory requirements spelled out in ASTM F432-13 to manufacture, measure and test the products can result in bad material, customer complaints and potential bolt failure with inadequate mine roof support.
What is needed is a cost effective rebar or bolt that can be manually bent when needed but that doesn't affect the material characteristics of the rebar.